PT symmetrical non-Hermitian system has attracted more and more attention [1]}. Recent works [2]}, [3]}, [4]}, [5]}, [6]}, [7]}, [8]}, [9]} have showed that exceptional point can enhance the sensitivity. It is mainly because that at exceptional point, two or more eigenvalues and their corresponding eigenvectors coalesce, leading to a nondiagonalizable Hamiltonian, which generates an Nth-order root law of eigenfrequency splitting.
